In an electrical machine the rotor rotates with an air gap between it and the stator. The rotor is most often fitted inside the stator, but solutions also exist in which the rotor is external to the stator or fitted with an air gap between it and the stator in the axial direction. The present invention is applicable to many different types of machines.
To the greatest extent possible, the air gap in the machine must be free of any dust and dirt coming from outside the machine and any impurities created in the machine during operation. Extra particles may affect the operating characteristics of the machine, such as noise and cooling, and may also cause direct or indirect damage to the machine's components. In particular, any impurities and particles of various sizes that have entered the air gap between the stator and rotor must be detected and removed before they cause damage to the stator or rotor.
The rotor of an electrical machine is supported on the stator body or end plate by bearings. These are also used to align the rotor with the centre of the stator so that the air gap is essentially equal in all positions. The rotor position may deviate from the centre line of the machine due to wear and tear on the bearings or potential bearing failure. In an extreme case this may lead to the rotor and stator coming into contact with each other.